専門家ではないので参考意見として；-
WikipediaのDinamicの項に、次のような説明があります。
Sudden changes：Sudden changes in dynamics are notated by an s prefixing the new dynamic notation, and the prefix is called subito. Subito is Italian as are most other dynamic notations, and translates into "suddenly".[3] It is usually used along with forzando (Italian for "forcing"), to make subito forzando, or what most people refer to as just sforzando. Other common uses of subito are before a regular dynamic notation, like in spp, sf, or sff.
ということで、おそらくsubitoの次に、何かくるかということではないでしょうか？
急にフォルテに(sf=sforzando)、急にフォルテッシモに(sff)、急にフォルテッシシーモに(sfff)等々。
